A Clearer View of HAT-P-1 b: JWST NIRSpec G395H Reveals Water, Carbon Dioxide, and Possibly Hydrogen Sulfide

@article{ashtari2026clearer,
  title = {{A Clearer View of HAT-P-1 b: JWST NIRSpec G395H Reveals Water, Carbon Dioxide, and Possibly Hydrogen Sulfide}},
  author = {Reza Ashtari and Stephen P. Schmidt and Guangwei Fu and Avinash Verma and David K. Sing and Kevin B. Stevenson and Jayesh Goyal and Katherine A. Bennett and Joshua D. Lothringer and Jacob Lustig-Yaeger and Sagnick Mukherjee and Carlos Gascón and Natalie H. Allen and Patrick McCreery and Le-Chris Wang and Mei Ting Mak and Kristin S. Sotzen and Lakeisha M. Ramos Rosado and N. J. Mayne},
  journal = {The Astronomical Journal},
  year = {2026},
  month = oct,
  doi = {10.3847/1538-3881/aea90a},
  url = {https://iopscience.iop.org/article/10.3847/1538-3881/aea90a},
  note = {CC BY 4.0},
}

TY  - JOUR
TI  - A Clearer View of HAT-P-1 b: JWST NIRSpec G395H Reveals Water, Carbon Dioxide, and Possibly Hydrogen Sulfide
AU  - Reza Ashtari
AU  - Stephen P. Schmidt
AU  - Guangwei Fu
AU  - Avinash Verma
AU  - David K. Sing
AU  - Kevin B. Stevenson
AU  - Jayesh Goyal
AU  - Katherine A. Bennett
AU  - Joshua D. Lothringer
AU  - Jacob Lustig-Yaeger
AU  - Sagnick Mukherjee
AU  - Carlos Gascón
AU  - Natalie H. Allen
AU  - Patrick McCreery
AU  - Le-Chris Wang
AU  - Mei Ting Mak
AU  - Kristin S. Sotzen
AU  - Lakeisha M. Ramos Rosado
AU  - N. J. Mayne
PY  - 2026
DA  - 2026/10/19
JO  - The Astronomical Journal
DO  - 10.3847/1538-3881/aea90a
UR  - https://iopscience.iop.org/article/10.3847/1538-3881/aea90a
AB  - As part of the James Webb Space Telescope (JWST) Exoplanet Grand Tour Survey, we use panchromatic transmission spectroscopy to connect HAT-P-1 b’s previously studied optical and near-infrared atmosphere to the longer-wavelength molecular bands accessible with JWST. We present JWST Near-infrared Spectrograph G395H transmission spectroscopy of the hot Jupiter HAT-P-1 b over 2.7–5.3 μ m, and combine the new spectrum with archival Hubble Space Telescope Space Telescope Imaging Spectrograph and Wide Field Camera 3 observations for a 0.3–5.3 μ m atmospheric analysis. We independently reduce the JWST data with the Eureka! , FIREFLy , and Tswift pipelines, finding mutually consistent transmission spectra across the G395H bandpass. Atmospheric retrievals yield strong evidence for H ₂ O and CO ₂ with Bayes factors of log 10 B H 2 O = 8.9 and log 10 B CO 2 = 52.3 , while providing tentative evidence for H ₂ S ( log 10 B H 2 S = 1.4 ). The joint H ₂ O and CO ₂ constraints favor an atmosphere near chemical equilibrium, with log 10 M/H = 0.9 9 − 0.14 + 0.19 , corresponding to ∼10× solar or ∼9× relative to the near-solar metallicity host star, and a 3 σ upper limit of C/O < 0.52. Because H ₂ O and CO ₂ provide a metallicity comparatively insensitive to vertical mixing in this temperature regime, their combined detection suggests the composition is dominated by bulk enrichment rather than strong disequilibrium transport. We find no significant evidence for clouds; instead, the persistence of molecular structure across the spectrum argues against strong cloud muting. The tentative H ₂ S signal, if confirmed, would further suggest limited photochemical processing at the pressures probed. Together, the molecular inventory, enriched metallicity, and low C/O ratio point to an oxygen-rich atmosphere and establish HAT-P-1 b as a benchmark for comparative studies of hot Jupiter atmospheric composition.
SN  - 0004-6256
SN  - 1538-3881
ER  -